The present invention relates to damping the motion used to position a magnetic tape head across the width of a magnetic tape.
Magnetic tape is commonly used to store voice and data information due to its reliability, cost efficiency, and ease of use. Magnetic tape may be made more useful and cost effective by increasing the areal density of information stored on the magnetic tape. This has generally been accomplished by including more data tracks on a given width of tape. While allowing more data to be stored, this increase in the number of data tracks requires a narrowing of the width of the data tracks, a narrowing of spacing between the data tracks, or both. As the data tracks are more closely spaced, positioning of the tape with respect to the tape head becomes more critical to reduce the possibility of errors introduced by reading or writing.
A tape head generally includes multiple read elements for simultaneously reading data from multiple data tracks and multiple write elements for simultaneously writing data to multiple data tracks. Typically, the number of data tracks is greater than the number of read or write elements. This requires the tape head to be properly positioned in a transverse direction across the width of the magnetic tape. Further, due to inaccuracies in data recording, tape geometry, and the tape path, the head position relative to the tape must be monitored and, if necessary, the head position modified to keep read and write elements centered over data tracks. To assist in head positioning, the head includes servo read elements that sense servo tracks interspersed with data tracks on the magnetic tape. The servo tracks include patterns that may be used to generate position error signals indicating the relative position of read elements and write elements to a particular set of data tracks. The servo tracks may also include patterns to identify over which set of data tracks read elements and write elements are currently located. The location of head elements over a particular set of data tracks is referred to as coarse positioning or track seeking while maintaining head elements centered over data tracks is referred to as fine positioning or track following.
There are many problems related to head positioning. One problem is the mechanism used to suspend the head. Typically, flexures such as a pair of simple cantilever springs or coil springs are used to suspend the tape head. An actuator, such as a voice coil motor, moves the head in the transverse direction against the opposing force of the flexures. In addition to transverse movement, the design of most flexures permit rotational head motion, translational motion normal to the transverse direction, or both. This results in a less than optimal position for some or all of the tape head elements.
A second problem is head motion damping. The head mass and flexures combine to produce an oscillatory system. When the tape head is moved, it will tend to exhibit decaying oscillation around a final position. In order to decrease head movement response time, damping is added to accelerate the rate of oscillation decay. Typically, purely mechanical damping is used. The damping may be provided by surrounding air, an additional viscous fluid, an elastomeric material connected between the head and the actuator, or the like. It is difficult to tune a purely mechanical system to compensate for mechanical variations in the head system and for changing operating conditions.
A third problem results from cabling used to carry electrical signals to and from the tape head. Typically, one or more cables connect the tape head to electronic cards which process read element signals and generate write element signals. These cables can exert forces on the tape head that impede proper head positioning. Further, electrical signals traveling on the cables can produce electromagnetic interference which may adversely affect signals on adjacent cables.
What is needed is a magnetic tape recording head system with improved positional accuracy and response time. The tape head should be restricted from any motion not in the transverse direction. The tape head should also be subjected to minimal forces from cabling connecting the head to read and write cards. Further, electrical signals traveling along cabling should not produce interference that adversely affects the operation of the tape system.
It is an object of the present invention to provide a system for accurately positioning a magnetic tape head across data tracks on a magnetic tape.
It is another object of the present invention to provide a system for damping undesirable oscillations that occur as a magnetic tape head is positioned across a magnetic tape.
It is still another object of the present invention to provide a system for accurately positioning a magnetic tape head across a magnetic tape that utilizes flexures restricting motion of the magnetic head in all directions except the transverse direction.
It is yet another object of the present invention to provide a system for accurately positioning a magnetic tape head across a magnetic tape having head interconnection cables that do not interfere with the positioning of the head.
It is a further object of the present invention to provide interconnection cables with minimal electromagnetic interference between cables and between any cable and electrical circuitry.
In carrying out the above objects and other objects and features of the present invention, a system for accessing magnetic tape is provided. The system includes a carriage frame and a magnetic tape head suspended from the carriage frame. A drive mechanism moves the tape head in a transverse direction relative to the carriage frame in response to a control signal. A linear velocity tachometer determines a velocity signal based on head movement in the transverse direction. Control logic receives a command to move the tape head, generates a control signal to change the head position, and modifies the control signal to dampen tape head oscillations relative to the carriage frame based on the velocity signal.
In an embodiment of the present invention, the magnetic tape head is suspended from the carriage frame by two flexures. The flexures permit the tape head to move in the transverse direction with respect to the carriage frame while restricting movement of the tape head in a direction parallel to tape travel and in a direction normal to the tape surface. In a refinement, each flexure includes a first spring member shaped like a rectangular frame, the rectangular frame defining a rectangular opening within the first spring member. A second spring member shaped substantially like a rectangular strip extends from a short side of the rectangular frame into the rectangular opening. The rectangular frame short side opposite the short side from which the second spring member extends is rigidly attached to the carriage frame and the rectangular strip short end opposite the short end extending from the rectangular frame is rigidly attached to the tape head.
In another embodiment of the present invention, the velocity tachometer includes a conductive sensing coil stationary relative to a magnetic field, the velocity signal based on the voltage produced by the sensing coil moving through the magnetic field. In a refinement, the velocity tachometer further includes at least one pair of magnets rigidly attached to the head frame and spaced so as to produce a magnetic field between the magnets in each pair. Each magnetic field has flux lines extending normal to the transverse direction. The conductive sensing coil is rigidly attached to the carriage frame and positioned to move between each of the at least one pair of magnets.
In still another embodiment of the present invention, the drive mechanism is a voice coil motor including a drive magnet rigidly attached to the tape head and a conductive motive coil rigidly attached to the carriage frame, the motive coil forming an opening for admitting the magnet. In a refinement, the voice coil motor further includes a cup having an open end. The cup is positioned around the drive magnet so as to admit the motive coil through the open end. The cup is constructed of a magnetically permeable material to reduce the effect on the tape head of a magnetic field produced by the drive mechanism.
The above objects and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.